The Wide Field of view Cherenkov Telescope Array (WFCTA) is an important part of the Large High Altitude Air Shower Observatory (LHAASO) project. WFCTA consists of 18 Cherenkov telescopes, and its main scientific objective is to measure the spectrum of cosmic rays. Each Cherenkov telescope is equipped with a large aperture spherical reflector consisting of 25 small spherical mirrors. In the process of cosmic ray detection and analysis, the reflectance of the telescope will directly affect the measurement of Cherenkov light. In the long-term use of mirror, the reflectance will experience varying degrees of attenuation. A reflectance measurement system of spherical mirror is presented in this paper. The system includes a light source and a measuring instrument for measuring the intensity of the reflected light. The multi-wavelength reflectance measurement system employed lasers with wavelengths of 405 nm, 450 nm, 488 nm, 520 nm, and 650 nm. The lasers exhibited fluctuation amplitudes of 0.06%, 0.01%, 0.02%, 0.01%, 0.06%, respectively. Additionally, the systematic errors when using different lasers were carefully assessed and found to be 0.2%, 0.2%, 0.1%, 0.2%, 0.3%, respectively. After performance tests in the laboratory, the measurement system was taken to on-site measure the reflectance of the spherical mirror of the telescope. A total of 175 different measuring points were selected for reflectance measurements, covering all 18 telescopes. The average reflectance is 78.5% (405 nm), 78.5% (450 nm), 78.5% (488 nm), 78.0% (520 nm), and 76.3% (650 nm). These measurements provide a reliable experimental basis for LHAASO-WFCTA.